In the manufacture of electronic or optoelectronic integrated circuits, an essential step is the patterning of a particular sequence of successive layers of insulative, semiconductive, conductive or optical material on a substrate. A common example is the patterning of a silicon dioxide (SiO.sub.2) layer formed on an epitaxial silicon substrate for the purpose of selectively removing the insulating layer to expose the underlying silicon. Typically, a thin film of organic "resist" material is formed over the SiO.sub.2 layer.
Next, a mask, comprising a transparent support material coated with a thin layer of opaque material engraved with the desired pattern, is placed on the resist surface. The engraved openings in the mask are located where it is desired to removed SiO.sub.2 to expose the Si substrate.
An intense beam of ionizing radiation from a source of ultraviolet light, or laser energy or X-rays, is projected onto the back surface of the mask and breaks down the molecular structure of the organic resist at openings in the mask. The exposed resist material is dissolved by immersing the wafer in a suitable solvent. In this manner, the opaque mask geometry is transferred onto the resist material on the SiO.sub.2 surface.
This resist pattern is now transferred to the SiO.sub.2, itself, by exposing the wafer to a material that will etch SiO.sub.2 but will not attack either the organic resist material or the Si substrate surface. This etching step is conventionally accomplished with hydrofluoric acid, which easily dissolves SiO.sub.2, but is incapable of etching or dissolving the organic resist. Next, the remaining organic resist is removed by a suitable organic, or acid, solvent.
The above process is a positive resist process in which the resist material remaining after exposure and development, corresponds to the opaque mask areas. Negative resists are also in common use.
The process in the above example, wherein the resist or SiO.sub.2 is etched away by an acid, is termed a wet-etching process. Wet etching processes are difficult to control because they lack sufficient anisotropic properties and, hence, result in an uncontrolled spread out of the etching width. Therefore, as requirements for patterning at the submicron level have developed, alternative anisotropic etching techniques have been developed using "dry" processes.
Anisotropic dry etching is usually accomplished with a combination of ions and chemically reactive species produced in a plasma from an unreactive gas. The ions are accelerated out of the plasma onto the material exposed through the resist where they initiate chemical reactions between the reactive species and the material forming volatile products. The directional nature of the ion flux is responsible for the anisotropic character of this etching technique.
Conventional dry anisotropic etching, used for semiconductor device fabrication, is accomplished by placing the material to be etched in a plasma. The plasma produces both ions and molecules with an incomplete bonding structure (radicals). The radicals are used to chemically react with the substrate forming volatile reaction products. The ions are used to initiate the reaction between the radicals and substrate. Since the ion flux is directional, the etching is substantially anisotropic.
While dry etching, as described above, is superior in many respects to wet etching, a number of disadvantages are present in the conventional systems. For example, ion bombardment is non-selective. Sputtering occurs. Thus, in addition to etching the SiO.sub.2 material, the ion bombardment can etch adjacent resist material and underlying Si material.
Also, the ion beam can damage adjacent or underlying material. Gate oxides can be damaged by electrons and UV light emissions. The ion beam equipment is relatively expensive and complex. The etching rates are relatively slow, i.e., less than 1 micron per minute. For these and other reasons, a need exists for an improved anisotropic dry etching process.